This may actually be influenced by a lot of things, but in my experience enabling the advanced features in Leopard Webkit (using the shell script included in its DMG file) shoud give you similar performance in QuickTime and in the browser HTML5 player. For a 360p h264 video I get around 70% overall CPU load both in QT and in the HTML5 player. With the advanced features disabled, or, of course, when using the old regular Safari, I get similar issues to the ones Imixmuan is experiencing, and full screen is smooth only when using QuickTime player.
eMac 1.25 GHz, 2GB ram, Leopard & Ubuntu 12.04 + XFCE.
C2D MBP 2,66 GHz, Mavericks